After watching one of the PE VFR workshops where you take off from Mongomery Gibbs KMYF, I noticed a bravo shelf to the north of the airport with two different bravo floors. Which floor altitude takes precedence? The 32/SFC or the 100/48? Attached a picture for reference.

Which takes precedence? They both do. There just happens to be a VFR corridor between them. Surface-3200 is one shelf, 4800-10000 is another. So...... You can be between 3200 and 4800 feet and not be in class B airspace.

The shelf you are referencing provides class B airspace, and required ATC services, from the SFC-3200' for protecting the departures off Miramar MCAS. The main shelf above MYF starts is 4800'-10000', and the referenced shelf above Miramar matches this 048-100. Both class B shelves must be followed, but this provides a class E cutout from 3201-4799' for VFR aircraft. Take a look at the San Diego Fly chart, which overlays light blue arrows for suggested VFR flyways. Notice how they cut the corner through that VFR cutout of the reference airspace.

For another example of this concept, take a look at the Class B airspace directly over SAN. It covers SFC-033 and again from 047-100. The VFR corridor routes are depicted by the magenta arrows, with their respective directions separated by magenta dotted line. Notice the route altitudes would be 3500' for south-eastbound. and 4500' for north-westbound aircraft, and the airspace provides a 200' buffer above and below these routes.